Pollution controlled polysulfide recovery process

ABSTRACT

A chemical recovery process for a polysulfide paper pulping system is described. After the polysulfide digestion, the pulp washing is carried out in an essentially oxygen-free environment to minimize the oxidation of NaSH. The spent liquor is preevaporated and mixed with a recycle stream of liquor containing Na2CO3 and NaHCO3. The mixture is next carbonated with a gas stream rich in CO2 which has been generated from lime kiln flue gas. The carbonated spent liquor is then vacuum stripped to release H2S which is converted to sulfur. The sulfur is mixed and reacted with Na2S in at least part of the white liquor to form sodium polysulfide. The polysulfide liquor and any remaining white liquor are returned to the digester. The pH of the desulfurized liquor from the vacuum stripping operation is increased to about 12.5 by the addition of make-up NaOH and recycled and oxidized white liquor. The resulting mixture is then evaporated to about 50 percent dry solids concentration. Finally, direct contact evaporation and combustion of the spent liquor occur as in conventional kraft chemical recovery systems. Means for recovering all sulfur compounds in vent gases and condensates from all mill process unite such as the digesters, evaporators, lime kiln, etc., are included to reduce air and water pollution.

United States Patent Fogmanet al.

451 Mai-.21, 1972 POLLUTION CONTROLLED Primary Examiner-S. Leon Bashore POLYSULFIDE RECOVERY PROCESS 2 g g g h R b L O] nomeyar ton ryan on ut er, en Inventors: Carl Bel'fil g Westfield. gh son, John F. Carney, Richard H. Berneike, Edward L. Kochey,

Wharton Nelson, West Hartford, Conn.; Jr nd Lawre e P Kessler Utah Tsao, Jersey City, NJ. [73] Assignees: Combustion Engineering, Inc., Windsor, [57] ABSTRACT Conn. by said Nelson; Lummus Company, A chemical recover recess for a olysulfide a er ul in Y P P P P P P g Bloomfield, NJ. by said Fogman and system is described. After the polysulfide digestion, the pulp Tsao washing is carried out in an essentially oxygen-free environment to minimize the oxidation of NaSH. The spent liquor is [22] June 1 1970 pre-evaporated and mixed with a recycle stream of liquor con- 2 1 APPL 45,455 taining Na CO and NaHCO The mixture is next carbonated with a gas stream rich in CO which has been generated from lime kiln flue gas. The carbonated spent liquor is then vacuum [52] US. Cl ..162/30, 23/48, 162/60 stripped to release 5 which is converted to lf The sulfur [51] Int. Cl ..D2lc 11/04 is mixed and reacted with s in at least part f the white [58] Field Of Search ..162/29, 30, 38, 4 J, 60; liquor to form sodium polysulfide, The polysulfide liquor and 136, 150 any remaining white liquor are returned to the digester.

. The H of the desulfurized li uor from the vacuum stri in R f Ct d P q PP g [56] e erences I e operation is increased to about 12.5 by the addition of make- UNITED STATES PATENTS up NaOH and recycled and oxidized white liquor. The resulting mixture is then evaporated to about percent dry solids 2,574,193 11/1951 Savell ..162/38 X concentration Finally, direct Contact evaporation and 2,999,044 9/1961 Collins bustion of the spent liquor occur as in conventional kraft 3,331,732 7/1967 Venemarkw M16250 chemical recovery systems. Means for recovering all sulfur 3,448,002 1969 Bryce at X compounds in vent gases and condensates from all mill 3,514,371 5/1970 Katsuo ..162/29 X process it ch as the digesters, evaporators, lime kiln,

V 7 etc., are included to reduce air and water pollution.

7 22 Claims, 4 Drawing Figures woov W-L- B- DIGESTER 1 1 icouaeuestz s'rizl P msl BLOW i i [a rowesz l4 TANK v VAPOR swueee f w i uquw WASH'NG ,Q, TANK TEAM- I l omsa arses FEE l l.

3a EVAP. *1

! a 42 44 SOAP iii "or & "I CAEBONATE /7a GKIMMER I E SYSTEM *H 62 C01 0E9- CAQBONATINS rowan 54 l HEATER 57$ #55 "QX1= i WEE i2 53 a3 1 5; A A M u 7. PM are? 222.2%, K (0 1 A55. Y 50 l, I a

OWER 5 57c FINAL. 86 SULP t EVAP- STOR A% E l 1%. 5a 1 2 l vnzscr I SULPWR 87 BLowsrz v74 5 ,5923? 1 mmamou I ATM. l r 1 l 60L eecovetzv *i Powsuu-m: {as SCIZUBBEIZ f72 FURNACE M1 REACTOR 1 7o 1 67 i LIM'E f smeur zv lc'g K'LN DIST- TANK fz TANK r\ 1 64 FUEL {68' CAUSTICIZER MUD WA sum J 66 cLARIFIEIZ POLLUTION CONTROLLED POLYSULFIDE RECOVERY PROCESS BACKGROUND OF THE INVENTION The pulping of wood by the polysulfide process is a known process that has been given much attention in recent years due to the higher yield of pulp from wood than obtained with the conventional kraft pulping process at the same pulp quality. Kraft pulping is done with an aqueous solution containing NaOH and Na S. In polysulfide pulping, sulfur is added to the same type of cooking liquor under conditions permitting the formation of sodium polysulfide, Na s," The improvement in pulp yield over the conventional process is on the order of to 14 percent and since raw wood is the major cost item in pulp production, the economic incentives to develop the process are obvious.

The initial sulfur charge in polysulfide pulping may be more than three times higher than that of kraft and the spent liquor cannot undergo normal kraft evaporation and combustion without serious consequences. As examples, there would be an intolerable level of H S release in the mill operation causing an air and water pollution problem and there would also be increased corrosion and decreased capacity in the multiple-effect evaporators. Also, there would be increased corrosion problems in the chemical recovery furnace as well as increased explosion hazards due to the high sulfidity. These problems plus sulfur losses would make the system impractical.

SUMMARY OF THE INVENTION The present invention is therefore directed to a practical chemical recovery system for use with polysulfide pulping. An object of the invention is to remove sulfur from the spent liquor prior to combustion in a chemical recovery furnace and to use this sulfur for producing fresh polysulfide cooking liquor. A further object is to prevent as much sulfur loss as possible thus preventing air and water pollution. These and other more detailed objects which will be more fully explained hereinafter are accomplished at least in part by means of the following process features.

The pulp washing after digestion is carried out so as to minimize the oxidation of NaSH which minimization is necessary to maximize sulfur recovery. The spent liquor is then preevaporated and mixed with a recycle stream containing NaH- CO and Na CO- so as to bring up the bicarbonate concentration to lower the pH for soap skimming and to bring up the carbonate concentration for the carbonation and H 8 stripping steps. Carbonation of the "spent liquor to form the bicarbonate results in some H S release. The bulk of the H S release takes place in the subsequent vacuum stripping step to produce a concentrated H 5 stream for conversion to elemental sulfur for the production of polysulfide cooking liquor. Other features of the invention include controlling the concentration and pH so as to minimize or avoid process difficulties and recovering sulfur-containing gas or liquid streams which otherwise might cause pollution. These and other objects and advantages will be more apparent from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall process flow diagram.

FIG. 2 is a flow diagram of the pulp washing portion of the system.

FIG. 3 is a flow diagram of the multiple-effect evaporator system.

FIG. 4 is a flow diagram of an alternate liquor recycle arrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Digestion Wood chips such as loblolly pine ships, polysulfide liquor and white liquor are fed to the digester 12 along with any recycled spent back liquor necessary for fill. The preferred amount of polysulfide sulfur in the cooking liquor mixture amounts to about 4 percent by weight (dry basis) of the wood. The maximum practical amount of polysulfide sulfur is about 7 percent and the invention is operable to any amount of polysulfide up to this maximum. The polysulfide digestion process may be carried out either in a batch or continuous type operation. The digestion products are fed to a blow or flash tank 14 in which vapors are released. The vapor streams from both the digester l2 and the blow tank 14 contain sulfur compounds primarily in the form of hydrogen sulfide and mercaptans. The quantity of released sulfur compounds would be considerably higher than for a kraft digestion process. These vapors are thereafter fed to one or more condensers 16 where condensibles are removed and then to a vapor sphere 18 for storage and further processing.

Pulp Washing In conventional kraft processes, the pulp undergoes a countercurrent washing operation to obtain a clean pulp product and a spent liquor at a relatively high concentration of dissolved nonpulp wood fractions and used chemicals. This washing is generally done in a threeor four-filter drum system having an open. exhaust hood for vapor outlet which permits air entry. The liquor in a pulp washer contains a considerable amount of NaSH. In an open hood washing operation, a substantial amount of this NaSH is oxidized to Na S O This is desirable in a kraft process since it fixes part of the sulfur in a nonvolatile form which does not produce H 5 in the evaporators downstream. The liquor coming from a polysulfide pulp process contains two to three times more NaSH than in the case of the kraft process. The successful operation of the present polysulfide sulfur recovery system requires that the amount of NaSH which is oxidized to Na S O be minimized. Otherwise, an unreasonably high amount of Na s would have to be present in the cooking chemicals in order to obtain sufficient NaSH for conversion to H 8 and subsequently to sulfur, since the Na- S O remains intact during the disclosed process and does not contribute to H S formation.

Therefore, the present invention employs a pulp washing system 20 as illustrated in detail in FIG. 2. This system includes a pulp washer 22 which is illustrated as having three filter drums. This pulp washer is totally enclosed by the hood 24 to essentially avoid any air infiltration. The filtrate from the drum washers is fed to individual filtrate tanks 26 in a countercurrent operation and the spent liquor discharged to the next operation as will be described hereinafter. The vapors from the filtrate tanks are fed to a foam tank 28 and then recycled by means of the blower 30 through vapor surge tank 32. As also illustrated in FIG. 2, the vapor surge tank 32 is connected by means of the surge line 34 with the polysulfide liquor tank 36. This is for the purpose of blanketing the polysulfide liquor in its storage tank with nitrogen and water vapor to prevent oxidation of polysulfide to inactive thiosulfate.

This totally enclosed pulp washing system would have to operate for an initial short period (1 2 hours) to exhaust the oxygen inventory of the washing system. The oxygen would be exhausted by reacting with some of the NaSH present during this initial period. After this period, the vapor would be essentially nitrogen and water vapor thus preventing further oxidation.

A secondary beneficial effect of enclosing the pulp washer is a reduced heat consumption. Generally hot water at 5560 C. is fed to the last filter drum as wash water. With an open hood design, flash evaporation of water from the liquor occurs which keeps the temperature of the filtrate liquor leaving the first filter drum at about C. At higher temperatures, the barometric leg suction principle applied to this type of washing operation would not function. With the totally enclosed system, such evaporation and heat loss would not occur. Therefore, in order to keep the temperature of the discharged spent liquor from going above 85 C., the wash water temperature must be reduced to about 3540 C. A further advantage of the enclosed washing system is the elimination of H 8 discharge to the atmosphere such as occurs in an open system. An additional benefit to be realized from enclosing the pulp washer is that the brightness of the pulp would be increased. The lignin still remaining undissolved in the pulp, ifoxidized in the presence of black liquor results in a reduction of the brightness. Elimination of oxygen prevents this from occurring and increases the pulp brightness.

Pre-evaporation The spent liquor from the pulp washing operation 20, after any necessary portion has been recycled to the digester 12, is fed to the pre-evaporators 38 which may consist, for example, of the last four stages of a six-stage multiple-effect evaporator system. The purpose of evaporation in a pulp mill chemical recovery system is to concentrate the organic and inorganic constituents so that the liquor may be burned in the chemical recovery furnace. The purpose of the pre-evaporation step at this point in the present invention is to create optimum conditions for two subsequent steps in the polysulfide sulfur recovery process, namely CO absorption and H 8 stripping. This pre-evaporation concentrates the NaSH and Na CO which increases the kinetics of the subsequent processing steps and reduces the size of the necessary equipment. Also, soaps can be effectively separated from the spent liquor after it has been pre-evaporated to 25-30 percent dry solids. This soap separation facilitates the following processing steps because of lessened foaming tendencies. The pre-evaporation also gives a relatively high liquor specific gravity, 1.12-1.15, and reasonably low viscosity, about 2 centipoise. The higher specific gravity results in the easier collapse of any foam that is generated and the viscosity of low enough for obtaining good chemical mass transfer in the CO absorption and H 5 stripping operations.

The pre-evaporation as well as the final evaporation sections are shown in more detail in FIG. 3. The function of the final evaporation section in the overall process will be described in more detail hereinafter. The weak black liquor from the pulp washing system at a concentration of about l5l 8 percent dry solids is fed to Stage VI of the countercurrent flow, multiple effect evaporator system. The vapor from this Stage VI, which contains some H 5, is fed to a condenser 40. The condensate C from the condenser is fed to a condensate treating system which will also be described hereinafter. The vented vapors from the condenser 40 are fed to a com pressor 42 as indicated in FIG. 1.

Stages lll through VI of the multiple effect evaporators are employed in the pre-evaporation section. The liquor flows from Stage VI to Stage V to Stage IV to Stage III and then out of the evaporator system to the soap skimmer 44 at a concentration of -30 percent dry solids. The vapors in the preevaporation section flow from Stage lll to IV to V and then to Stage Vl. Minor continuous vapor purge from the steam chests ofStages IV, V, and V! are made to release H S through the lower part of the steam chests of the evaporator bodies. The effluent from these steam chests is conducted to condenser with the condensate being fed to the above-mentioned condensate treating system and with the H S-containing vented vapors being fed to compressor 42 along with the vapors from Stage VI. The compressor 42 is used for receiving the vented vapors from the condenser 40. lmproved Soap Skimming and Recycle The effluent from Stage III of the pre-evaporation section is fed to a conventional soap skimmer 44 which provides approximately one hour dwell time as in normal skimming operation. The 25-30 percent dry solids concentration of this feed to the soap skimmer is most favorable to such a skimming operation. It has been found, however, that the normal soap skimming operation can be greatly improved by recycling a quantity of liquor rich in NaHCO from the subsequent carbonation step as shown in FIG. 1. This recycle has two beneficial effects. Firstly, the NaHCO lowers the pH which enhances soap separation. Secondly, the Nal-lCO; reacts with NaSH forming H 8 bubbles, which facilitate flotation of soap to the surface. The amount of carbonated liquor recycled is adjusted to obtain the necessary soap separation. It has been found that a recycle of between about 50 and 200 percent of the incoming main liquor stream is a suitable range with about percent recycle being preferred.

An alternate recycle scheme is illustrated in FIG. 4. In this arrangement, the recycle is H 8 stripped liquor taken from the vacuum stripper rather than carbonated liquor from the carbonating tower. This recycle is passed through a recarbonator 49 in which Na CO is converted to NaHCO The CO supply to the recarbonator 49 may be a 100 percent CO stream (dry basis) such as from a hot carbonate system, which will be described hereinafter, or it may be a flue gas stream such as from a lime kiln. This recarbonation step provides the NaH- CO in the recycle stream necessary for effective soap skimming as described above.

The soap skimming operation in the present invention has several advantages. IN passing spent liquor containing soaps through a packed column, there is a tendency for the soaps to coat the packing material. This soap coating forms a sticky surface to which the precipitated lignin readily adheres. This can very rapidly cause the packed column to plug. Effective soap removal can essentially eliminate this problem. Another problem which occurs when soaps are present in spent liquor is foaming in various operations such as gas-spent liquor contact steps including carbonation, stripping, and oxidation. This foaming problem can also be reduced by means of the improved soap skimming operation. A third benefit of the improved soap skimming is that the soap yield and consequently the tall oil yield if significantly increased.

Carbonation and Recycle Following soap skimming, the main liquor stream is passed to the carbonating tower 50 wherein CO is reacted with Na CO to produce NaHCO according to the following reactron:

Na CO CO H 0 ZNaHCO (I) The H 8 stripping step, which will be explained hereinafter involves a reaction of this NaHCO with NaSH to form H 5 according to the following reaction:

NaSH NaHCO H 8 Na C0 2) The following side reaction occurs simultaneously in the vacuum stripping tower:

2NaHCO Na CO +CO H 0 (3 This means that there must be sufficient Na CO present in the feed to the carbonating tower to subsequently provide sufficient NaHCO for the two latter reactions. Since the spent liquor feed to the carbonating tower may not contain enough Na Co for this purpose, a quantity of spent liquor from the subsequent l-l S stripping step which is rich in Na CO; can be recycled and introduced into the carbonating tower 50 along with the main liquor stream. The amount of recycle is selected such that the mole ratio of NaHCO to Na CO at the end of the carbonating step will be at least 7.5 and the mole ratio of NaHCO to NaSH will be about 1.5 2.5. Depending upon the amount of Na CO in the incoming liquor stream to the carbonation tower 50, this recycle may be anywhere from 0 100 percent of the quantity of liquor coming from the preevaporation section. When these conditions exist, there will be sufficient NaHCO available to permit the subsequent stripping of H 8 and CO according to reactions (2) and (3) above.

The CO gas stream fed to the carbonating tower 50 is preferably 100 percent CO (dry basis) at about the tower operating pressure and temperature. This pure CO gas stream is obtained from a hot carbonate system which will be described hereinafter and it is passed countercurrent to the downcoming liquor stream. It is desirable that only enough CO be passed through the carbonating tower 50 to produce the desired amount of NaHCO The fact that the CO stream is concentrated and thus small, in terms of gas flow, means that the amount of H 8 released in the carbonating tower will be small because of its partial pressure/vapor pressure relabonating tower would be much higher. The discharge would be a gas stream lean in H S. But the purpose of this invention is to create a mixture of gases from the carbonating tower, the H 8 stripping tower and other sources so rich in H 8 as to permit an efficient operation of the Clause reactor 84.

The amount of CO fed to the carbonating tower should be as follows:

CO in the gas feed NaSH in the liquor feed expressed as molar ratio.

Although the carbonating tower 50 may operate, for example, at a pressure anywhere from atmospheric to 100 p.s.i.a.- and at a temperature from 25-l00 C., a more practical range is atmospheric pressure to 30 p.s.i.a. and 5080 C. As the temperature is increased, the partial pressure of CO decreases and the vapor pressure of the CO increases so that the pressure differential from the gas to the liquid would become nil. The temperature should also not get too low since the liquor becomes too viscous and the reaction rates become too low. The overhead vapor stream from the carbonating tower 50 containing excess CO and some H S is combined with the vent from condenser 54, the discharge from compressor 42, and the vapor sphere discharge stream as part of the feed to the Claus reactor 84.

H S Stripping A portion of the carbonated liquor stream discharged from the carbonating tower 50 is recycled to the soap skimmer 44 as discussed hereinbefore. The remaining carbonated liquor stream is fed to the vacuum stripping tower 51 together with a bleed stream from the hot carbonate system which will be explained hereinafter. This bleed stream contains sodium carbonate and bicarbonate (approximately 12 to 15 percent) and a small amount of Na SO (approximately 5 to 6 percent). The bleed stream can also be introduced before the carbonating tower 50.

'l'hc stripping tower 51 is a packed column in which the hicarbonatcd liquor undergoes vacuum steam stripping whereby H 8 and some CO are released. Any other type of equipment conventionally used for gas stripping may be used in place of the packed column. The stream for carrying out this stripping operation is obtained, for example, from Stage I of the multiple effect evaporator system as indicated in FIG. 3 or from stages II or III. The primary reaction that occurs is:

(2) NaSH NaHCO; Na C H This would require one mole of NaHCO for one mole of NaSH but there is acc mpeti r g ide reactionz 0.5 4.0 (preferably 1 2) 2NaHCO Na CO +'H. .o' C(I This side reaction requires that additional NaHCO be present and furthermore, a residual amount of NaHCO must be present as a chemical driving force according to the following equilibrium equation:

(HCOQ'XSIF) At constant K and (SH), the higher the ratio of bicarbonate to carbonate, the higher the vapor pressure of H 8 over the liquid to be stripped. The liquor temperature in the stripping tower will be at or below the boiling point of the liquor at the tower pressure. The vacuum stripping tower may operate, for example, at a pressure anywhere from about 5 in. Hg absolute up to atmospheric pressure. A more practical range is 8-17 in. Hg absolute with the preferred pressure being about 10 in. Hg absolute. The overhead vapor stream from the stripping tower is condensed to overhead condenser 52, compressed by vacuum pump 53 to atmospheric pressure, fed to condenser 54 and then fed to the sulfur recovery system along with other vented vapor streams.

8 Chemical Makeup Themakeup chemical in a kraft mill is generally Na SO, in which the ratio of sodium and sulfur losses (Na,:S) is equal to one. In the polysulfide pulping and recovery system, the sulfide sulfur contained in all liquor streams is substantially higher than in the case of kraft. In order to abate the emission of sulfur compounds, practically every vapor and condensate stream is captured and elemental sulfur recovered for reuse in the process resulting in a chemical loss ratio (Na,:S) of about 1.4. To achieve pollution and chemical process control as described hereinafter, sodium hydroxide is used as a makeup for sodium losses and is added to the main liquor stream after the H 8 stripping step. Makeup sulfur is added to the elemental sulfur stream after Claus reactor 84 as explained hereinafter. Therefore, the sulfidity of smelt from the recovery boiler is kept at a low level reducing the odor and smelt explosion problems from this particular unit.

The liquor coming from the stripping tower 51 has a pH of about l0.0-10.5 and the addition of NaOH will increase the pH. However, the amount of NaOH added for sodium makeup reasons may not be sufiicient to increase the pH to a range in which the precipitated lignin will be dissolved and kept in solution through the subsequent evaporation steps. Otherwise, lignin precipitate would foul the surfaces of the evaporators. Therefore, in order to increase the pH to the desired range of 12-13, a quantity of white liquor is recycled as shown in FIG. 1. This increased pH will also prevent the release of H 5 in the following direct contact evaporation step.

The amount of white liquor recycled and the exact pH desired will depend primarily upon the type of direct contact evaporator which is used. If the evaporating gas stream is hot flue gas, which contains CO that lowers the liquor pH, it will be necessaryv to add more white liquor to give a higher initial pH. If hot air is used in the direct contact evaporator, in which case the pH will not be lowered, less white liquor can be recycled and perhaps the recycle can even be eliminated. Oxidation The recycle of white liquor as described above will introduce some Na S into the black liquor stream in addition to whatever residual Na s might be present. This Na S may be partially converted to H 8 in a flue gas direct contact evaporator and as such would be released to the atmosphere as a pollutant. Therefore, the black liquor, after the white liquor addition is subjected to a low level oxidation step in which the Na S is oxidized to the stable Na S O This step is carried out with air or oxygen in the oxidizing tower 56 which may be a packed tower or any other suitable form of liquid gas contactor. Black liquor oxidizers have been used in the past but with soap-containing liquors they have failed due to foaming. In the present invention, however, the soaps have been removed and foaming is no longer a serios problem. air from the oxidizing tower 56, which may contain some sulfur compounds, is incinerated in the recovery boiler as shown in FIG. 1.

Final Evaporation The H 3 stripped liquor after mixing with the makeup NaOH and any necessary white liquor recycle and after oxidizing tower 56 is then fed to Stage II of the multiple-effect evaporator system as shown in FIG. 3. The final evaporators 57 increase the concentration of the liquor up to 50-55 percent dry solids. The vapors from Stage II of the final evaporation section are used to heat Stage III of the pre-evaporator section while the vapors from Stage I of the final evaporation section are used both to heat Stage II and to feed the vacuum stripping tower 51 as previously mentioned.

Direct Contact Evaporation The liquor discharged from Stage I of the final evaporation system is fed to a direct contact evaporator such as a conventional cascade evaporator in which water is evaporated from the liquor by direct contact with the flue gases from the recovery boiler. Another type of direct contact evaporator which can be used is the air cascade evaporator in which air, which has been heated by heat exchange with the flue gases, is used for the evaporation instead of flue gases. In this case, the liquor would not be contacted with the flue gas CO and its pH would not be lowered. Therefore, as described above, it might be possible to reduce or eliminate the white liquor recycle. With the air cascade evaporator, the oxidizing tower 56 can also be eliminated.

The liquor is concentrated in this evaporator from 50-55 percent to about 70 percent dry solids concentration producing a liquor suitable for burning. Due to the low content of NaSH and Na s in the liquor at this point and the high pH, the emission of H 8 from the direct contact evaporator by reaction with CO will be close to nil. Under certain conditions H S discharged from the recovery furnace could be partially absorbed in this evaporator contributing to the reduction in air pollution by H S.

Recovery Furnace The liquor from the direct contact evaporator is next fed to a conventional chemical recovery furnace 60 with steam boiler discharging a smelt consisting mainly of Na S and Na CO and a flue has which generates steam in the boiler section of the furnace. The sulfidity of the smelt is about 25 percent, which is normal as compared to conventional kraft units. High sulfur-to-sodium ratio in the feed liquor to a chemical recovery furnace causes corrosion problems and greatly increases the chances of dangerous smelt-water explosions. A sulfidity of about 25 percent is significantly below the danger level. The flue gases from the chemical recovery furnace are used either directly or indirectly to carry out the evaporation in the direct contact evaporator as indicated above and in FIG. 1.

Smelt Processing The smelt from the chemical recovery furnace is handled in much the same manner as in the conventional kraft process. The smelt is dissolved in an aqueous solution in tank 62 producing what is commonly called the green liquor. The green liquor from the smelt dissolving tank together with cal- Cium oxide from the lime kiln is then reacted in the causticizer 64 to form conventional white liquor which is then clarified at 66. The white liquor from the clarifier is drained off into the white liquor storage tank 67 and a portion recycled to the H 8 stripped black liquor stream as needed and discussed above. The solid precipitate from the clarifier 66 containing mostly CaCO is conducted to the washer 68 in which the CaCO is washed. The wash water is then used to dissolve the smelt in tank 62. The CaCO from the washer is next fed to the lime kiln 70 in which the CaCO is converted to CaO and C The CaO from the lime kiln 70 is then recycled to the causticizer 64.

The gases from the lime kiln 70 containing primarily CO and N are passed to a wet scrubber 72 for the removal of particular matter and the major part of any SO present and then passed by means of the blower 74 into the hot carbonate systemv The hot carbonate system comprises a C0 absorption tower 76 and a C0 desorption tower 78. The absorbing liquor in the hot carbonate system is a mixture of NaHCO and Na CO The gases as discharged from the absorption tower 76 to atmosphere do not contain any H 8 or SO thus eliminating this pollution problem. The liquor from the absorption tower 76 is then sent via a heater 80 to the CO desorption tower 78 in which CO is stripped off the bicarbonate/carbonate mixture at a temperature of about 220230 F. This desorption tower has a reboiler at the bottom to furnish the necessary heat for the stripping operation. The liquor from the desorption tower, which is then cooled, is recycled to the absorption tower 76. The overhead vapor from the desorption tower 78 contains 100 percent CO (dry basis) and is used to feed the carbonating tower 50. Since there will be a tendency to build up Na SO and other matter in the hot carbonate system, a portion of the circulating liquor in this system is bled and fed to the carbonation tower 50 or the stripping tower 51 as previously discussed. Pure NaOH is used as makeup for the circulating liquor in the hot carbonate system as a replacement for the bleed stream.

Sulfur Recovery The vapors from the vacuum stripping tower 51, the carbonation tower 50, the pre-evaporators 38 and the vapor sphere 18 are mixed as shown in FIG. 1. The mixture containing about 50 percent H S and 50 percent CO by volume is further compressed at 82. The gas from the compressor 82 is passed to a reactor 84 in which the hydrogen sulfide is converted to elemental sulfur. This conversion may be accomplished by any one of the many processes available. One such process is the well-known Claus reaction, a gas phase, catalytic reaction, which takes place at relatively high temperature. The gas from the compressor 82 at a pressure of about 9 p.s.i.g. is combined with air also at 9 p.s.i.g. in the exact stoichiometric ratio and reacted in the Claus reactor as follows:

H S+1-%O SOgl'HgO 4 2H S S0 3S 2H O 5) The tail-end gases from the Claus reactor contain primarily CO and N but may contain some unreacted H 5 and 80,. These gases are therefore sent as part of the air feed to the chemical recovery furnace for complete incineration of the H S and S0 At the temperatures prevailing in the zone of air introduction, volatilized Na O will combine with S0 to form Na SO This will be further oxidized to Na SO by the oxygen in excess air. This Na SO is removed from the flue gases by the conventional electrostatic precipitator. The elemental sulfur from the Claus reactor is discharged to the sulfur storage 85. The exothermix heat from the Claus reactions can be utilized for steam generation. Polysulfide Production The molten sulfur from the sulfur storage 85 together with molten makeup sulfur is subjected to cooling, prilling and crushing in the sulfur preparation section 87. The sulfur is then metered along with the white liquor to the polysulflde reactor system 88 in which the mixture is circulated between a mixer and settler during which the sulfur is reacted with sodium sulfide in the white liquor to produce polysulfide according to the following reaction:

Na S+nS Na S,,+ m wherein n is normally 1-4, preferably 1.0-2.5. The amount of polysulfide produced in this conventional reaction will depend upon the various proportions of sulfur to white liquor and various operating conditions in the digester 12. The polysulfide liquor is then fed to tank 36 for storage under oxygen-free blanket in preparation for use in pulping.

Pollution Control Sulfur compounds are generated in a polysulflde pulpproducing mill at a rate five to six times larger than in a kraft mill. This requires that measures be taken to prevent air and water pollution. This is accomplished according to the present invention by collecting all such streams and recovering the sulfur compounds. For example, the gases from the digester 12, the blow tank 14, the pre-evaporators 38, and the carbonating tower 50, after removing the condensibles are sent to the Claus reactor 84 for the conversion of sulfur compounds into elemental sulfur. The condensates C" from condenser 16, 40, 48, 52 and 54 are sent to a condensate recovery system which includes an H 5 stripping tower 90. The combined condensate is heated and stripped at 212 F. with steam, yielding a sulfur-free reusable bottoms stream and an overhead vapor. This overhead vapor is condensed and the condensate is decanted yielding raw turpentine and water both containing sulfur compounds. The turpentine fraction can be sold as such on burned in the recovery boiler 60, while the water fraction is recycled to the stripping tower. The vent leaving the condenser joins the digestion vents in the vapor sphere 18 for sulfur conversion in the Claus reactor 84.

It can be seen from the above description of the invention that the problems associated with the chemical recovery of polysulfide spent liquor such as high sulfldity in the chemical recovery furnace, sulfur losses', and air and water pollution, have been overcome.

While the foregoing description has specifically referred to the production of pulp from loblolly pine, it will be apparent that the present invention is fully applicable to the treatment of other cellulose fiber materials such as all the species of wood used in the pulp industry as well as the annual crops in the form of bagasse, straw and the like.

While preferred embodiments and alternatives of the invention have been shown and described, it will be understood that they are merely illustrative and that changes may be made without departing from the scope of the invention as claimed.

We claim:

I. A method of processing the NaSH- and Na CO -containing spent liquor from a polysulfide pulping process comprising the steps of:

a. washing polysulfide spent liquor from the digested pulp in a substantially oxygen-free atmosphere whereby oxidation of NaSH is minimized;

b. adding a recycle stream containing NaHCO to said liquor thereby reducing the pH;

. carbonating said spent liquor with CO so as to convert Na CO to NaHCO whereby said NaHCO will be present in an amount sufficient to react with substantially all of said NaSH to form H 3;

. vacuum stripping said H S from said spent liquor at a pressure below atmospheric;

e. increasing the pH of said stripped spent liquor to a level which will prevent lignin precipitation during subsequent processing including the step of adding NaOH for sodium makeup;

. evaporating and processing said spent liquor to produce white liquor containing NaOH and Na S;

g. converting said H 3 to elemental sulfur; and

h. reacting said elemental sulfur with at least a portion of said white liquor to produce polysulfide cooking liquor.

2. A method according to claim 1 and further including the step of pre-evaporating said spent liquor subsequent to washing step (a) and prior to NaHCO addition step (b).

3. A method according to claim 2 wherein said recycle stream containing NaHCO added in step (b) is a recycle stream of carbonated spent liquor from step (c).

4. A method according to claim 3 and further including the step of adding Na CO to said spent liquor prior to said carbonating step (c), said added Na C being contained in a recycle stream of vacuum stripped spent liquor from step (d).

5. A method according to claim 2 wherein said recycle stream containing NaHCO added in step (b) is a recycle stream of vacuum stripped spent liquor from step (d) and further including the step of recarbonating said recycle stream to convert at least a portion of the Na CO in said recycle stream to NaHCO 6. A method according to claim 2 wherein said spent liquor is pre-evaporated to a dry solids content of 25-30 percent.

7. A method according to claim 6 wherein said carbonating step (c) takes place'at a temperature of from about 25 to 100 C. and at a pressure of between about atmospheric pressure and I00 p.s.i.a.

8. A method according to claim 7 wherein said vacuum stripping step is carried out at a pressure of between about in. Hg absolute and atmospheric pressure.

9. A method according to claim 8 wherein said carbonating temperature is from about 50 to 80C. and said carbonating pressure is between about atmospheric pressure and 30 p.s.i.a.

10. A method according to claim 9 wherein said stripping pressure is between about 8 17 in. Hg absolute.

11. A method according to claim 1 wherein the step (e) of increasing the pH of said stripped spent liquor further includes the step of recycling a quantity of white liquor to increase the pH of said stripped spent liquor above the point attained by said NaOH addition.

12. A method according to claim 11 and further including the step of oxidizing Na S in said spent liquor subsequent to the step of recycling said white liquor and prior to step (f).

13. A method according to claim 1 wherein said carbonating step (c) comprises contacting said spent liquor with a gas stream which is substantially pure CO on the dry basis.

14. A- method according to claim 13 wherein said processing step (f) comprises concentrating said spent liquor by evaporation, burning said spent liquor to produce a smelt containing Na CO and Na s and processing said smelt to produce said white liquor and additionally to produce a C0 containing gas stream and further including the step of processing said Co -containing gas stream to produce said substantially pure CO for step (c).

15. A method of processing the spent liquor from a polysulfide pulping process wherein said liquor contains NaSH and Na CO comprising the steps of:

a. separating the pulp from said liquor and conducting said liquor to a pre-evaporation stage while maintaining said liquor in a substantially oxygen-free atmosphere to minimize the oxidation of said NaSH;

b. pre-evaporating said liquor to about 25-30 percent solids;

c. adding a NaHCo -containing recycle stream to said liquor to lower the pH;

d. skimming soap from said liquor;

e. contacting said liquor with a Co -containing gas stream which is substantially pure CO on the dry basis, so as to convert Na CO to NaHCO whereby said NaHCO will be present in an amount sufficient to react with substantially all of said NaSH to form H S,'said contacting step being carried out at a temperature of from about 50 to C. and at a pressure of between about atmospheric pressure and 30 p.s.i.a. whereby said CO and said Na CO and said NaSH react to form said NaHCO as well as a gaseous effluent containing a small quantity of H 5;

. subjecting said liquor to vacuum stripping whereby H 8 gas formed by reaction of NaHCO with NaSH is separated from said liquor, said vacuum stripping being carried out at a pressure between about 5 in. Hg absolute and atmospheric pressure;

g. increasing the pH of said stripped spent liquor to a level which will prevent lignin precipitation during subsequent processing including the step of adding NaOH for sodium makeup;

h. concentrating said liquor by evaporation to produce a liquor capable of burning;

i. burning said concentrated liquor to provide a smelt containing Na CO and Nags;

j. processing said smelt to produce a white liquor containing NaOH and Na S and to produce a CO -containing gas;

k. processing said Co -containing gas to provide said substantially pure CO on the dry basis for step (e);

l. collecting the H S gaseous effluents from steps (b) and (e) and (f) and converting said H 5 to elemental sulfur;

m. reacting said elemental sulfur with at least a portion of said white liquor to produce polysulfide cooking liquor. 16. A method according to claim 15 and further including the step of adding a Na CO -containing recycle stream to said liquor subsequent to skimming step (d) and prior to contacting step (e).

17. A method according to claim 15 wherein said NaHCO recycle added in step (c) is a recycle stream of vacuum stripped liquor from step (f) and further including the step of recarbonating said recycle stream to convert at least a portion of the Na CO in said recycle stream to NaHCO 18. A method according to claim 15 wherein said step (1) further includes collecting the gaseous effluents containing sulfur compounds from other of said processing steps and converting said sulfur compounds to elemental sulfur.

l9. A-method according to claim 1 and further including the step of separating soap from said spent liquor.

20. A method according to claim 19 wherein said soap is separated from said spent liquor subsequent to NaHCO addition step (b) and prior to said carbonating step (c).

21. A method according to claim 1 wherein said mole ratio of NaHCO to NaSH subsequent to carbonating step (c) is about 1.5 to 2.5.

22. A method according to claim 21 wherein the mole ratio of the CO employed in carbonating step (c) to the amount of NaSH in the liquor is about 0.5 to 4.0. 

2. A method according to claim 1 and further including the step of pre-evaporating said spent liquor subsequent to washing step (a) and prior to NaHCO3 addition step (b).
 3. A method according to claim 2 wherein said recycle stream containing NaHCO3 added in step (b) is a recycle stream of carbonated spent liquor from step (c).
 4. A method according to claim 3 and further including the step of adding Na2CO3 to said spent liquor prior to said carbonating step (c), said added Na2CO3 being contained in a recycle stream of vacuum stripped spent liquor from step (d).
 5. A method according to claim 2 wherein said recycle stream containing NaHCO3 added in step (b) is a recycle stream of vacuum stripped spent liquor from step (d) and further including the step of recarbonating said recycle stream to convert at least a portion of the Na2CO3 in said recycle stream to NaHCO3.
 6. A method according to claim 2 wherein said spent liquor is pre-evaporated to a dry solids content of 25-30 percent.
 7. A method accOrding to claim 6 wherein said carbonating step (c) takes place at a temperature of from about 25* to 100* C. and at a pressure of between about atmospheric pressure and 100 p.s.i.a.
 8. A method according to claim 7 wherein said vacuum stripping step is carried out at a pressure of between about 5 in. Hg absolute and atmospheric pressure.
 9. A method according to claim 8 wherein said carbonating temperature is from about 50* to 80* C. and said carbonating pressure is between about atmospheric pressure and 30 p.s.i.a.
 10. A method according to claim 9 wherein said stripping pressure is between about 8 - 17 in. Hg absolute.
 11. A method according to claim 1 wherein the step (e) of increasing the pH of said stripped spent liquor further includes the step of recycling a quantity of white liquor to increase the pH of said stripped spent liquor above the point attained by said NaOH addition.
 12. A method according to claim 11 and further including the step of oxidizing Na2S in said spent liquor subsequent to the step of recycling said white liquor and prior to step (f).
 13. A method according to claim 1 wherein said carbonating step (c) comprises contacting said spent liquor with a gas stream which is substantially pure CO2 on the dry basis.
 14. A method according to claim 13 wherein said processing step (f) comprises concentrating said spent liquor by evaporation, burning said spent liquor to produce a smelt containing Na2CO3 and Na2S and processing said smelt to produce said white liquor and additionally to produce a CO2-containing gas stream and further including the step of processing said CO2-containing gas stream to produce said substantially pure CO2 for step (c).
 15. A method of processing the spent liquor from a polysulfide pulping process wherein said liquor contains NaSH and Na2CO3 comprising the steps of: a. separating the pulp from said liquor and conducting said liquor to a pre-evaporation stage while maintaining said liquor in a substantially oxygen-free atmosphere to minimize the oxidation of said NaSH; b. pre-evaporating said liquor to about 25-30 percent solids; c. adding a NaHCO3-containing recycle stream to said liquor to lower the pH; d. skimming soap from said liquor; e. contacting said liquor with a CO2-containing gas stream which is substantially pure CO2 on the dry basis, so as to convert Na2CO3 to NaHCO3 whereby said NaHCO3 will be present in an amount sufficient to react with substantially all of said NaSH to form H2S, said contacting step being carried out at a temperature of from about 50* to 80* C. and at a pressure of between about atmospheric pressure and 30 p.s.i.a. whereby said CO2 and said Na2CO3 and said NaSH react to form said NaHCO3 as well as a gaseous effluent containing a small quantity of H2S; f. subjecting said liquor to vacuum stripping whereby H2S gas formed by reaction of NaHCO3 with NaSH is separated from said liquor, said vacuum stripping being carried out at a pressure between about 5 in. Hg absolute and atmospheric pressure; g. increasing the pH of said stripped spent liquor to a level which will prevent lignin precipitation during subsequent processing including the step of adding NaOH for sodium makeup; h. concentrating said liquor by evaporation to produce a liquor capable of burning; i. burning said concentrated liquor to provide a smelt containing Na2CO3 and Na2S; j. processing said smelt to produce a white liquor containing NaOH and Na2S and to produce a CO2-containing gas; k. processing said CO2-containing gas to provide said substantially pure CO2 on the dry basis for step (e); l. collecting the H2S gaseous effluents from steps (b) and (e) and (f) and converting said H2S to elemental sulfur; m. reacting said elemental sulfur with at least a portion of said white liquor to produce polysulfide cooking liquor.
 16. A method according to claim 15 and further including the step of adding a Na2CO3-containing recycle stream to said liquor subsequent to skimming step (d) and prior to contacting step (e).
 17. A method according to claim 15 wherein said NaHCO3 recycle added in step (c) is a recycle stream of vacuum stripped liquor from step (f) and further including the step of recarbonating said recycle stream to convert at least a portion of the Na2CO3 in said recycle stream to NaHCO3.
 18. A method according to claim 15 wherein said step (l) further includes collecting the gaseous effluents containing sulfur compounds from other of said processing steps and converting said sulfur compounds to elemental sulfur.
 19. A method according to claim 1 and further including the step of separating soap from said spent liquor.
 20. A method according to claim 19 wherein said soap is separated from said spent liquor subsequent to NaHCO3 addition step (b) and prior to said carbonating step (c).
 21. A method according to claim 1 wherein said mole ratio of NaHCO3 to NaSH subsequent to carbonating step (c) is about 1.5 to 2.5.
 22. A method according to claim 21 wherein the mole ratio of the CO2 employed in carbonating step (c) to the amount of NaSH in the liquor is about 0.5 to 4.0. 